1. Filed of the Invention
The present invention relates to an optical fiber chromatic dispersion distribution measuring apparatus for measuring the chromatic dispersion distribution of an optical fiber and a measuring method.
2. Description of the Related Art
It is known that when two pulse light beams having different wavelengths xcex1, xcex2 from each other are simultaneously inputted to an optical fiber under test, four-wave mixing light beams are generated due to interaction between the two inputted pulse light beams.
A relation among the wavelengths xcex1, xcex2 of the pulse light beams and wavelengths xcex3, xcex4 of the four-wave mixing light beams is shown in FIG. 2.
In FIG. 2, the longitudinal axis indicates the wavelength of each of light beams and the transverse axis indicates the intensity of each of light beams. Symbols a and b indicate the pulse light beams having the wavelength xcex1 and xcex2, respectively. Symbols c and d indicate the four-wave mixing light beams having wavelengths xcex3 and xcex4, respectively. The wavelengths xcex1, xcex2, xcex3, and xcex4 satisfy the following relation:
xcex1xe2x88x92xcex3=xcex4xe2x88x92xcex2=xcex2xe2x88x92xcex1=xcex0 (xcex0 is about 5 to 10 nm) 
An interval between the wavelengths of the pulse light beams (that is, xcex2xe2x88x92xcex1=xcex0) is the smaller, the intensity of the four-wave mixing light beams are the larger.
An optical fiber chromatic dispersion distribution measuring apparatus according to a related art extracts either one of the four-wave mixing light beams having the wavelengths xcex3 and xcex4 that are reflected from the optical fiber under test, by an optical bandpass filter having a variable center wavelength to execute measurement of the chromatic dispersion distribution of the optical fiber under test.
However, due to a mechanical structure of the optical bandpass filter having the variable center wavelength, a loss caused by inserting the optical bandpass filter having the variable center wavelength is more than 10 dB to decrease the measurement sensitivity.
In case of compensating the loss, which is caused by inserting the optical bandpass filter having the variable center wavelength, by using an optical amplifier, the configuration of the apparatus becomes complicate.
As shown in FIG. 2, the backscattered light beam of the four-wave mixing light beams measured by an optical time domain reflectometer (OTDR) is varied in accordance with a distance periodically. The backscattered light beam has a relation that the variation period is in proportional to the dispersion value. Therefore, the dispersion value is estimated from the variation period.
In a wave form shown in FIG. 2, a wave form X having a pulse shape indicated by a dotted line is supposed to be displayed on the optical time domain reflectometer as a far end of an optical fiber under test. However, since the obtained backscattered light beam of the four-wave mixing light beams varies periodically, the part X having pulse shape indicated by the dotted line is not clearly specified by the measuring apparatus according to the related art as shown in FIG. 2.
Consequently, the far end of the optical fiber under test is not clearly determined from the display output of the optical time domain reflectometer (OTDR). Therefore, it is problem that the far end of the optical fiber under test is difficult to be specified.
The reason for need to specify the far end of an optical fiber under test a test fiber is that the length and the refractive index of the optical fiber have such a relationship that if one parameter is known, the other can be calculated automatically. In order to know the refractive index of the optical fiber, it is necessary to know the length of the optical fiber.
It is an object of the invention to provide an optical fiber chromatic dispersion distribution measuring apparatus enabling to change interval between two wavelengths inputted and having the high sensitivity with a simple configuration and to provide an optical fiber chromatic dispersion distribution measuring apparatus which can easily determine a far end of the optical fiber under test from display of an optical time domain reflectometer (OTDR).
In order to solve the above described problem, according to a first aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measuring apparatus comprising:
two light sources for outputting light beams having different wavelengths from each other, respectively, to an optical fiber under test;
an optical time domain reflectometer for measuring four-wave mixing light beams generated by an interaction between the light beams inputted to the optical fiber under test;
an optical bandpass filter having a fixed center wavelength; and
a coherence controller for controlling coherence of at least one of the outputted light beams of the two light sources,
wherein at least one of the two light source is a tunable light source; and
the optical bandpass filter is disposed at a previous stage of the optical time domain reflectometer. Whereby the spectral line width is widened to be able to easily determine the far end of the optical fiber under test from the display output of the optical time domain reflectometer (OTDR).
According to a second aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measuring apparatus comprising:
two light sources for outputting CW light beams having different wavelengths from each other, respectively;
a coherence controller for controlling coherence of at least one of the CW light beams of the two light sources;
an optical coupler for combining a plurality of light beams;
a modulator;
an optical fiber amplifier;
a directional coupler;
an optical fiber under test;
an optical bandpass filter having a fixed center wavelength; and
an optical time domain reflectometer;
wherein at least one of the two light source is a tunable light source;
the two light sources output the CW light beams to the optical coupler;
the optical coupler combines the CW light beams and outputs the combined CW light beams to the modulator;
the modulator modulates the CW light beams inputted from the optical coupler to generate pulse light beams having different wavelengths from each other and outputs the pulse light beams to the optical fiber amplifier;
the optical fiber amplifier amplifies the pulse light beams and outputs the amplified pulse light beams to the directional coupler;
the directional coupler outputs the pulse light beams inputted from the optical fiber amplifier to the optical fiber under test and outputs a light beam inputted from the optical fiber under test to the optical bandpass filter;
four-wave mixing light beams are generated in the optical fiber under test due to an interaction between the pulse light beams inputted from the directional coupler and is outputted to the directional coupler;
the optical bandpass filter extracts a light beam within a specific band from the light beam inputted from the directional coupler and outputs the extracted light beam to the optical time domain refelectometer; and
the optical time domain reflectometer measures the chromatic dispersion distribution of the extracted light beam.
According to a third aspect of the invention, there is provided the apparatus according to the second aspect of the invention, wherein the four-wave mixing light beams are a light beam generated in lower frequency side than the pulse light beams and a light beam generated in higher frequency side than the pulse light beams; and
only one of the four-wave mixing light beams is within the specific band of the optical bandpass filter. Therefore, the four-wave mixing light beams can be selected on a basis of a relation with regard to the center wavelength of the bandpass filter.
According to a fourth aspect of the invention, there is provided An optical fiber chromatic dispersion distribution measuring method comprising the steps of:
outputting two light beams having different wavelengths from each other, respectively, to an optical fiber under test;
controlling coherence of at least one of the light beams;
generating two four-wave mixing light beams in the optical fiber under test;
measuring one of the two four-wave mixing light beams to obtain the chromatic dispersion distribution of the optical fiber under test. Whereby, the spectral line width is widened so that the far end position of the optical fiber can be easily determined from the display output of the optical time domain reflectometer.
According to a fifth aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measurement method comprising the steps of:
outputting two CW light beams having different wavelengths from each other;
controlling coherence of at least one of the CW light beams;
combining the CW light beams;
modulating the CW light beams to generate two pulse light beams having the different wavelengths from each other;
amplifying the pulse light beams;
inputting the pulse light beams to an optical fiber under test to generate two four-wave mixing light beams;
extracting one of the four-wave mixing light beams; and measuring the one of the four-wave mixing light beams to obtain the chromatic dispersion distribution of the optical fiber under test.
According to a sixth aspect of the invention, there is provided the method according to the fifth aspect of the invention, further comprising the steps of adjusting both wavelengths of the two light beams so that wavelength of the one of the four-wave mixing light beams coincides with a center wavelength of an optical bandpass filter having a fixed center wavelength for executing the extracting step. Therefore, position of the far end of the optical fiber can be easily determined by using the normal optical time domain reflectometer (OTDR) without using a bandpass filter having a variable center wavelength.
According to a seventh aspect of the invention, there is provided the method according to the fifth aspect of the invention, wherein ratio of the intensity of the two CW light beams is approximately 2:1. Whereby, the optical fiber wavelength dispersion can be measured without any measurable variations in frequency under observation.